I haven't checked all of them, but I'm pretty sure all UK SIPP providers will hold (uninvested) client money in trust, or at least in segregated accounts. Here are some examples:
All client money is held by us on trust and is segregated from our own
funds in accordance with the FCA’s client money rules and guidance so
that any creditors of Hargreaves Lansdown would have no legal right to
it and we cannot use any of this money to cover Hargreaves Lansdown's
Any money (cash) held in your account is treated as ‘client money’ as
defined by the FCA. This means your money also has trust status, and
is deposited across a range of bank accounts specially designated as
holding client money. At no point does your cash enter an ii bank
(interestingly doesn't use the phrase "in trust")
Investment firms such as Fidelity are very different from banks
because we are required to separate client money and assets from our
own resources. We are not permitted to use client money and assets in
the course of our own business activities, and your money would be
ring-fenced in the unlikely event that we became insolvent.
In general a site-specific search for the term fscs will get you to a provider's 'reassurance' page(s).